Glyphosate Application Timing Influences the Tolerance of Two Glyphosate-Tolerant Perennial Ryegrass Cultivars.

Glyphosate application timing influences the tolerance of two glyphosate-tolerant perennial ryegrass cultivars

Christian M. Baldwin, A. Douglas Brede, Jami J. Mayer, and R. Golembiewski

Utilizing glyphosate-tolerant perennial ryegrass (Lolium perenne L.) (PRG) cultivars, ‘JS501' and ‘Replay', provide turfgrass managers a unique option for annual bluegrass (Poa annua L.) (ABG) control.  Regarding tolerance, both cultivars can tolerate a maximum glyphosate rate of 0.81 kg ae (acid equivalent) ha^-1 when applied to an established stand and under optimal growing temperatures.  However, tolerance levels following applications during seed germination and low temperatures remain unknown.  Therefore, objectives of this research were to determine the seedling tolerance and low temperature response following a late season glyphosate application of both cultivars.  Field trials were conducted in ID and OR.  For the low temperature response trial in ID, glyphosate was applied at 0, 0.15, 0.29, 0.58, 1.16, 1.74, 2.32, and 3.48 kg ae ha^-1.  In OR, glyphosate was applied at 0, 0.15, 0.29, 0.44, 0.58, 1.16, and 3.48 kg ae ha^-1.  At both sites, applications were made between late September and early October.  To determine seedling tolerance, both cultivars were sprayed with glyphosate at the 1 leaf stage (LS), 2LS, 3LS, or 4LS at rates of 0, 0.15, 0.29, and 0.58 kg ae ha^-1.  Across all trials, ratings included PRG color, cover, and injury.  At both trial locations, regression analysis revealed a rate of 0.19 kg ae ha^-1 was required to cause 20% leaf firing in the low temperature response trial.  In the seedling tolerance trial, glyphosate applied at 0.58 kg ae ha^-1 at the 1LS, 2LS, and 3LS had color ratings ≥ 8.0, however, color ratings dropped to 4.6 when an application was made at the 4LS.  Based on the environmental conditions of each trial, results suggest glyphosate applications greater than 0.15 kg ae ha^-1 as minimum air temperatures approach 0^oC should be avoided.  Also, applications should be avoided at the 3 to 4LS if the application rate is greater than 0.29 kg ae ha^-1.